methods in which variables are indexed with a subscript indicating the time period in which the observation occurred. For example, y_t might refer to the value of income observed in unspecified time period t, y₃ to the value of income observed in the third time period, etc.

Moreover, when a researcher attempts to develop a theory to explain what is observed in discrete time, often the theory itself is expressed in discrete time in order to facilitate the development of a time series